Many wavelength tunable light sources include a resonant optical cavity that includes an optical gain element and one or more filter elements. The optical resonant optical cavity quantizes light oscillation to a discrete set of evenly-spaced optical modes most of which are quenched by the filter elements. In many applications, it is desirable to produce a single-wavelength output beam in a single optical mode so that the light source may be tuned continuously without mode hopping over a specified range of frequencies. To achieve this result, the optical modes of the resonant cavity and the frequency response of the filter elements of the light source must be tuned simultaneously. In addition, the ratio of the filter bandwidth to the optical mode spacing should be relatively small to achieve high mode stability.
Some types of wavelength tunable light sources use a moving mirror grating to tune the light sources to produce an output beam of a particular wavelength. For example, in one approach, the so-called “Littrow” configuration, an optical cavity is formed between a facet on a semiconductor laser and a diffraction grating. A collimating lens focuses a light beam generated by the laser onto the grating. In the cavity formed by the facet and the grating there is only a single wavelength (i.e., the “Littrow wavelength”) of light that exactly satisfies the condition that the angle of light incident on the grating be the same as the angle of light diffracted from the grating. In this way, only light at or very near the Littrow wavelength oscillates between the facet and the grating. The Littrow wavelength is changed by simultaneously rotating the grating and moving the grating toward or away from the facet.
Other types of light sources use tunable acousto-optic devices to produce an output beam of a particular wavelength. An acousto-optic device typically includes a transducer for generating acoustic waves in an optical medium. Light injected into the optical medium interacts with the acoustic waves to modify characteristics of the injected light. For example, in an acousto-optic modulator, acoustic waves propagating in the optical medium are used to modulate the intensity of the injected light. In an acousto-optic deflector, the acoustic waves deflect the injected light beam by an amount that varies with the acoustic frequency. In an acousto-optic tunable filter, only injected light within a narrow wavelength passband is converted from one polarization to another polarization and a polarization filter selectively passes light with the converted polarization. The wavelength passband is determined by the acoustic frequency applied to the acousto-optic deflectors.
Each of the above-mentioned acousto-optic devices induces a Doppler frequency shift in the beam, where the direction of the shift depends on the propagation direction of the acoustic waves with respect to the light beam. The Doppler frequency shift induced by a single acousto-optic device accumulates as the light beam oscillates in the resonant cavity of a light source, preventing single-frequency operation of the light source. For this reason, many acousto-optic-based light sources include at least one pair of acousto-optic devices that produce substantially offsetting Doppler frequency shifts.
In one approach, a pair of acousto-optic devices is used in a laser or other optical resonator to produce a wavelength-dependent deflection of the light without incurring a net frequency shift. In this approach, the dispersive quality of acousto-optic devices in transmission is used as a reflection grating substitute. Two mirrors, one an output coupler, a gain medium and two acousto-optic devices are arranged for maximum efficiency such that the incident and diffracted beams are approximately at the Bragg angle for each acousto-optic device. In this way, the output wavelength is determined by the acoustic frequency of the acousto-optic devices.
In another approach, an external cavity laser is formed by feeding light from a laser through two or more acousto-optic tunable filters and back to the laser. The Doppler shifts introduced by the filters do not entirely cancel. The net Doppler shift caused by the filters is continuously tuned to cause the bandpass of the filter to follow the shifting resonating frequency of the laser to reduce mode hopping.